The use of inductors and transformers in electrical circuits is widely known. Different types of design exist. In a much-used design, the inductor comprises a spiral winding of an electrically conductive wire, for instance a copper wire. When current passes such winding, an electromagnetic field is generated, having field lines that basically follow the winding's center line and form closed loops by curving back outside the winding, so that the field lines in longitudinal cross section resemble the shape of an 8. The flux of the field is proportional to the current magnitude I. The energy stored in the electromagnetic field can be expressed as E=0.5·L·I2, with L being the inductance of the inductor. For increasing the inductance of a given winding, or for reducing the volume of the inductor while maintaining the inductance, it is known to use a core of a high permeability material to guide and concentrate the field lines. It is possible to have a bar core in the center of the winding, but it is also possible to have a core that follows the entire length of the field lines and thus has a cross-sectional shape resembling the cipher 8.
FIGS. 1A and 1B are schematic longitudinal cross-sections illustrating two implementations of a conventional design of an inductive component 10, 20 including a soft magnetic core. In this respect, reference is made to Chapter 26 “Passive Components and Practical Converter Design Considerations” in Power Electronics: Converters, Applications, and Design by N. Mohan, T. M. Undeland, and W. P. Robbins, John Wiley & Sons, New York, 1989.
Soft magnetic cores usually consist of two core components separated by air gaps. In the case of FIG. 1A, the two core components 11 can be mutually identical and have a general E-shape with two outer legs 12, 13 and a central leg 14 connected together by a bar 15. The winding 16 is made on a plastic bobbin 17, and then the two core components 11 are placed together with their respective legs 12, 13 facing each other and with the bobbin 17 in between. The central legs 14 are slightly shorter than the outer legs 12, 13, so that the outer legs can touch each other or have only a very small air gap between them, while a larger air gap 18 is present between the central legs 14. The air gaps in the outer legs are negligibly small whilst the air gap 18 in the central leg 14 is intentionally made larger.
It is noted that soft magnetic cores are available in a wide variety of shapes, sizes, and magnetic materials. Likewise, bobbins are also available in a wide variety of shapes and sizes. It is further noted that the winding typically consists of a wire made from copper or aluminium. The wire may be solid wire, but Litz wire and foil are also used. The bobbin is provided with metal pins to which the terminals of the wire are soldered. These metal pins are used to mount the inductor on a printed circuit board via surface-mount or through-hole connection.
In the case of FIG. 1B, one core component 21 is implemented as an inner drum while the other core component 22 is implemented as an outer cylinder shell fitting around the drum. The drum 21 has a general I-shaped cross section, having a central body 23 and two outer discs 24, 25 having a diameter larger than the central body 23; this can also be described as a cylindrical drum 21 having a circumferential groove or recess 26. The inner diameter of the cylinder shell 22 is larger than the outer diameter of the drum 21, so that air gaps 28, 29 exist between the shell 22 and the respective outer discs 24, 25. The winding 27 is made in the groove 26 of the drum 21, and then the cylinder shell 22 is applied around the drum 21.
In the examples of FIGS. 1A and 1B, the inductive component 10, 20 comprises one single winding 16, 27. In such case, the component is an inductor. It is also possible that the inductive component comprises two or more windings arranged in the same bobbin 17 or the same groove 26, respectively: in such case, the two or more windings are magnetically coupled. Such design is useful for instance for implementing two or more coupled inductors, or for implementing a transformer.
It is generally desirable to reduce the volume of electronic components. This applies especially to inductors, as they tend to be the larger components in an electronic circuit. It applies most specifically to electronic circuits that need to be mounted in small spaces, for instance LED retrofit lamps. LED retrofit lamps are lamps that are intended to be mounted to replace for instance an incandescent lamp, and even may have an outer appearance resembling an incandescent lamp, but internally they comprise one or more LEDs and an LED driver. In a typical example, an LED retrofit lamp comprises a bulb portion and a bayonet-type base or Edison screw-type base having an LED driver arranged in the base. Such LED driver receives standard AC mains, in Europe typically 230 VAC @ 50 Hz, and must convert the AC mains voltage to DC LED current. A converter type widely used in such driver is a switch-mode converter. It is noted that switch-mode converters are known per se so that an explanation of the design of a switch-mode converter will be kept brief. By way of example, reference is made to parts 2, 3 and 5 in Power Electronics: Converters, Applications, and Design by N. Mohan, T. M. Undeland, and W. P. Robbins, John Wiley & Sons, New York, 1989.
FIG. 2 is a simplified circuit diagram illustrating some of the basic components of a possible implementation of a two-stage switch-mode converter.
The switch-mode converter 30 of FIG. 2 comprises a rectifier stage 31 for rectifying the mains voltage Vac. A boost PFC stage 32 comprises a first inductor 34, a first diode D1 and a first switch 51. The first switch 51 is controlled to maintain an intermediate voltage Vi at a buffer capacitor 36. A buck DC/DC converting output stage 33 converts the DC intermediate voltage Vi to DC load current. This output stage 33 comprises a second switch S2 and a second inductor 35 in series with the load L, and a second diode D2 parallel to the load L and the second inductor 35. The load is here shown as a resistor 37 but this will in LED retrofit lamps be replaced by one or more LEDs.
The key issue as far as the present invention is concerned, is that such switch-mode converter comprises two (or more) inductors, which makes the desirablility to reduce the volume of the inductors even more stringent.
Usually, each inductor is built as a separate component, i.e. a separate entity. It is already known that it is possible to reduce the combined volume of two inductors if these two inductors are built as one combined component, including two separate windings on one common drum core. FIG. 3 is a schematic longitudinal cross section comparable with FIG. 1B, illustrating the general design of such dual inductive component.
Comparable to the inductive component 20 of FIG. 1B, the dual inductive component 40 of FIG. 3 comprises one core component 41 implemented as a cylindrical drum while the other core component 42 is implemented as a cylinder shell fitting around the drum. When the windings are in place, the cylinder shell 42 is applied around the drum 41, extending the full length of the drum 41. Different from the embodiment of FIG. 1B, the cylindrical drum 41 has two spaced apart circumferential grooves or recesses 43, 44, with respective windings 45, 46 arranged in the respective grooves. The cylindrical drum 41 has a first outer disc 47, a second outer disc 48, and a central disc 49 between the two grooves 43, 44. A first air gap 51 is present between the first outer disc 47 and the cylinder shell 42, a second air gap 52 is present between the second outer disc 48 and the cylinder shell 42, and a central air gap 53 is present between the central disc 49 and the cylinder shell 42.